1. Field of the Invention
The present invention relates to a vehicle motion control apparatus, particularly relates to an apparatus for performing at least one of controls including a steering control, braking force control and throttle control, to maintain a vehicle stability.
2. Description of the Related Arts
In order to maintain a stability of a vehicle, there is known heretofore an apparatus for setting a desired vehicle behavior, comparing the same with an actual vehicle behavior to provide a deviation between them, and controlling a braking force or steering angle on the basis of the deviation, as disclosed in Japanese Patent Laid-open Publication Nos. 2-70561 and 2-106468, for example. And, there is disclosed in Japanese Patent Laid-open Publication No. 62-146754, an apparatus for setting a front wheel speed difference and a target value of lateral acceleration or yaw rate, on the basis of a steering angle and vehicle speed, to control brake and/or engine outputs.
In any of the Publications as described above, the apparatuses relate to a closed loop control of vehicle behavior, and operate in a vehicle limit zone. In order to improve a vehicle stability in motion, therefore, an operation zone for vehicle stability control is required to extend to a normal operation zone. According to the apparatuses, however, the vehicle stability control is performed on the basis of the vehicle behavior as a state variable of the vehicle. As a result, there is a limit to a control zone, which is to be extended to the normal operation zone.
The above-described subject matter regarding the operation zone and control zone will be explained hereinafter, referring to FIG. 2 schematically showing a characteristic of side force applied to a tire. The side force to the tire (wheel) is increased linearly, with a wheel slip angle increased, and saturated at a limit of coefficient of friction against a road. For example, when the road coefficient of friction μ is high, the side force characteristic is the one as indicated by 0-S-T in FIG. 2. If the road coefficient of friction μ is decreased, the characteristic will be the one as indicated by 0-Q-R in FIG. 2. As the state variable indicative of the vehicle behavior, such as lateral acceleration or yaw rate, directly reflects the side force characteristic of the tire, the characteristic of the state variable will be similar to the one as shown in FIG. 2.
According to the closed loop control of the vehicle behavior, the limit of coefficient of friction can be determined on the basis of the vehicle behavior in the limit zone, such as a point (X) in FIG. 2. However, if the vehicle is in such a state as indicated by a point (Y), it is impossible to determine whether the point (Y) is on the side force characteristic of 0-S-T (i.e., high-μ characteristic), or on the side force characteristic of 0-Q-R (i.e., low-μ characteristic), as shown in FIG. 2. In other words, by means of only the state variable indicative of the vehicle behavior, it is impossible to determine whether there would be a margin to the limit of the coefficient of friction against the road surface, or the vehicle could be approaching an unstable zone.